Yesterday, the World Health Organisation (WHO) released what appeared to be a routine statement about four brothers in Pakistan who had all contracted the H5N1 strain of 'bird flu'.

However, the statement revealed that three of the four had had no known contact with sick or dead poultry.

The incident has again raised fears of a bird flu pandemic.

The H5N1 virus was first discovered in 1997, but human cases of the illness have been limited.

If the virus gains the ability to pass easily between humans the results could be catastrophic.

Dr Margaret Chan, the director-general of the WHO, tells Sir David that bird flu poses an emormous threat to human health across the world and that while it is difficult to predict if a pandemic will occur, scientists are closely monitoring the virus.

Zimbabweans are still awaiting the outcome of last week's presidential elections.

Robert Mugabe's ruling Zanu-PF has already lost its majority in parliament for the first time since independence in 1980.

The opposition Movement for Democratic Change (MDC) claim their leader, Morgan Tsvangirai, has won the presidential election.

However, it looks likely that there will be a run-off vote.

There is concern that a re-run could inflame tensions in the country and lead to violence.

The elections come against a backdrop of increasing economic troubles in the former British colony. Inflation is running at more than 100,000 per cent and fuel and food shortages are commonplace.

Knox Chitiyo of the Royal United Services Institute joins Sir David to discuss the situation and says that if a run-off were to occur, Morgan Tsvangirai would be the firm favourite to win by a decisive margin.

He adds that if Tsvangirai is elected there will be an enormous sense of expectation that he may find hard to fulfill.

James Miles

James Miles was one of thefew journalists in Tibet when the violence began

After experiencing some of its worst violence for years, in which more than 100 people were reportedly killed, the province of Tibet is once again calm.

It is thought the violence began after a number of Buddhist monks were arrested in the Tibetan capital Lhasa.

Most Tibetans dispute China's claim to the Himalayan territory and resent the increasing economic, political and cultural dominance of the Chinese.

With the Beijing Olympics taking place later this year the issue of Tibet's sovereignty is likely to remain in the international spotlight.

James Miles, the China correspondent for The Economist, joins Sir David to discuss the tensions.

Miles was one of the few journalists in Tibet when the violence began.

He says the scale of the violence was unprecedented in China and adds that despite the justified grievances of ethnic Tibetans, without the Chinese, economic growth in the region would be seriously hindered.

There are growing fears that global food supplies cannot keep up with demand.

Food prices have risen by 75 per cent over the last three years and there have been food riots in some countries, including India and Mexico.

High oil prices have pushed up transportation and processing costs, demand from India and China, and the United State's dash for ethanol are among many factors that have contributed to the price rises.

Sir David is joined by Simon Maxwell of the Overseas Development Institute, and by Francis Moore-Lappe of the Small Planet Institute.

Maxwell says governments have to respond by providing more social security, safety nets and famine relief.

Moore-Lappe says that hunger is not a result of a scarcity of food but a scarcity of democracy.

Frank Lutz, an American pollster, joinsSir David to discuss the presidential race

The race for the Democratic presidential nomination in the US was always going to be fiercely contested.

However, few predicted it would last this long. What started in the January snows of Iowa looks set to run through to the final contests in Montana and South Dakota in June.

It could even go beyond that all the way to the August convention in Denver.

Barack Obama currently leads his Democratic rival Hillary Clinton in the delegate count but not by an insurmountable margin.

Meanwhile John McCain, the confirmed Republican nominee, has already set about establishing his presidential credentials for the big election in November.

Sir David is joined by American pollster Frank Luntz to discuss the presidential race.

Luntz says that despite Clinton's strong established support, Obama is the likely winner of the Democratic nomination; adding that if this happens a segment of Democratic voters are likely to vote for McCain.